The electron probe micro-analyzer (EPMA) can obtain quantitative compositional information from micro-volumes. The technique is generally non-destructive, and in-situ, such that compositions can be related to textural relationships of phases in the sample of interest. In geological specimens, this typically means the analysis of petrographic thin sections, but other samples can be loaded as well. The samples should be flat and well-polished to yield accurate results. EPMA is typically employed for major and minor element analysis, but can be used for trace elements down to the ppm level under certain circumstances. The UMass facility specializes in high spatial resolution trace element EPMA and geochronology.
Acquired compositions are used to establish micro-chemical relationships that can, in turn, be used to construct reaction histories that trace the evolution of the sample. In some cases, the measurement of actinides and radiogenic lead can be used to estimate in-situ ages at very high spatial resolution in geological specimens. Quantitative microanalysis establishes phase compositions in spatial context (including the characterization of growth zoning and diffusion profiles) that can be used ultimately to address many problems in the physical sciences, here are a few examples:
Lithospheric processes such as rates and histories of crustal tectonics, volcanism, sedimentary provenance, metamorphic histories
Planetary formation processes in the early solar system through analysis of meteorites, moon rocks, and ancient rocks preserved from the early Earth
Geochemical fractionation processes for understanding mineral resource development and environmentally important geochemical issues
Engineering research applied to metallurgy, glasses, ceramics, ceramic superconductors, zeolites, optical fibers, semiconductors, solders, filter media
Biomineralization, fish ecology, crustacian disease studies, processes of fossilization
Forensics
General principles
Many signals are produced when a focused electron beam interacts with a specimen…
Beam electrons scatter elastically and inelastically, with inelastic scattering imparting energy to the target atoms. The dimensions of the interaction volume are therefore determined by the initial beam energy and the characteristics of the specimen (density, average atomic number). We can therefore say that electron scattering processes ultimately result in energy transfer to atoms in the target, and a statistically definable interaction volume that represents the practical analytical spatial resolution. Energetic beam electrons entering the specimen will interact, lose energy and change direction, many eventually falling to near-zero energy levels:
Inelastic interactions induce various processes, including the generation of characteristic X-rays from the target. These X-rays, resulting from ionization of inner shells and subsequent relaxation to the ground state, have wavelengths (and energies) characteristic of the electronic structure of the target atoms. They can therefore be used to fingerprint atoms, and the beam current normalized characteristic X-ray count rate can be related to elemental concentration.
Additionally, Bremsstrahlung radiation, resulting from beam electron energy loss in the coulomb fields of the target atoms will produce an X-ray background which must be subtracted from the raw characteristic X-ray intensity to give a net intensity representing the true contribution of the element of interest.
The X-ray path as it exits the specimen, undergoes intensity modification due to absorption and secondary fluorescence. Additionally, the incident beam undergoes intensity modification due to interaction with the specimen. These processes must be corrected to produce a true quantitative result, which is initially estimated by the ratio of a characteristic X-ray intensity to one produced from a standard of known composition, for example for silicon:
wt.fraction Si = ISiK? (unknown) / ISiK? (pure std.)
K-ratio = [ISiK? (unknown) / ISiK? (std.)] x Cstd
Cstd relates concentration in std to pure element
K x 100 = uncorrected wt.%
Once these uncorrected values are obtained for all elements present in the sample, they can be corrected for the above mentioned “matrix effects” to give an accurate result.
X-ray detection can be done by either energy dispersive (EDS) or wavelength-dipersive spectrometry (WDS). WDS techniques that distinguish EPMA (see below) have the advantage of high spectral resolution, high peak to background (arising primarily from bremstraahlung), and high count rate fidelity. This is achieved through the use of Bragg diffraction, wherein the X-rays emitted from the sample are diffracted into a gas flow proportional counter using a synthetic monochromator of known d-spacing, in some cases yielding energy resolution of nearly 1 eV. Different elements are acquired by moving the spectrometer to the appropriate characteristic wavelength.
The primary advantage of EDS is that the entire X-ray spectrum (up to the beam energy) is instantaneously acquired. This allows rapid evaluation of unknowns to establish, at least qualitatively, the elemental make-up of the sample. Both types of detectors are available on the EPMA platform.
